Isolation of gamma benzene hexachloride



United States Patent ISOLATION OF GAMMA BENZENE HEXACHLORIDE Keith J. Smith and James S. Sconce, Niagara Falls, N. Y., assignors to Hooker Electrochemical Company, Niagara Falls, N. Y., a corporation of New York No Drawing. Application March 1, 1948, Serial No. 12,466

8 Claims. (Cl. 260--648) The present invention relates to a novelmethod for the recovery, in substantially pure form, of. ai s'pecific 2,699,455 Patented Jan. 11, 1955 "ice its mixtures with other isomers. Such separation is deor to enhance the proportion of 'y is'mer in other mixtures. The insecticidally active isomer, 'y-1,2,3,4,5,6- hexachlorocyclohexane, is often referred to as gammexane (Slade, Chemical Trade Journal and Chemical Engineer for March 16, 1945, page 279, Canadian Chemistry and Process Industries, January 1946, page 30).

SEPARATION OF THE ISOMIERS Separation of isomers of hexachlorocyclohexane is not broadly new. As stated above, Meunier in 1887, showed that hexachlorocyclohexane contained two isomers, the a and ,6. F. E. Matthews (J. Chem. Soc. 59, 165 (1891)) disclosed a method for roughly separating the oz and ,B isomers by steam distillation, followed by purification and concentration by recrystallization from alcohol or isomer of hexachlorocyclohexane from products such as INTRODUCTION Hexachlorocyclohexane, often called benzene hexachloride" or 666, and the various isomers thereof have been known for some time. Michael Faraday appears to have been the first to prepare hexachlorocyclohexane, obtaining his product by the chlorination of bi-carburet of hydrogen (benzene) in sunlight. Mitscherlich described sirable to obtain the pure 'y for use as an insecticide per se, acetic acid, as early as 1891. In his publication of 1912, van der Linden (Berichte 45, 231 (1912)) repeated the work of Matthews and reported substantially the same results, also disclosing the 'y and 6 isomers. The separation of these isomers likewise involved the employment of ether, alcohol and/or acetic acid. After van der Lindens work, interest in the separation of the isomers ap- 5 pears to have waned for several decades.

the decomposition of hexachlorocyclohexane to trichlorobenzene by means of bases in 1833. Peligot and Laurent settled the composition of hexachlorocyclohexane in 1836. Meunier in 1887 showed that hexachlorocyclohexane contained two isomers, the ct, melting at 157 degrees centigrade, and the B, melting at a much higher temperature. In 1912, van der Linden established the existence of four isomers, the a, ,6, 'y, and 5. Since that time other workers have established the existence of the e isomer.

Processes for the production of hexachlorocyclohexane have been described in United States patents to Bender 2,010,841 and to Hardie 2,218,148, and in British Patents 447,058 and 504,569. Still another process for chlorinating benzene to produce hexachlorocyclohexane containing an enhanced proportion of the 'y isomer is disclosed in application for United States patent of Stormon, Serial 749,366, filed May 20, 1947, now U. S. Patent; 2,499,120. Crude hexachlorocyclohexane produced in accordance with the processes of the above disclosures may, contain the various isomers in the following percentages:

Isomer: Per cent Alpha-hexachlorocyclohexane 60-75 Beta-hexachlorocyclohexane j:. 5-12 Gamma-hexachlorocyclohexane 8l6 Delta-hexachlorocyclohexane 5-12 Epsilon-hexachlorocyclohexane 1-3 Heptachlorocyclohexane and other impurities 05 Of the various isomers of hexachlorohexane, only the 'y isomer has been shown to possess insecticidal activity of a usable degree. This 7 isomer, the active principle of any hexachlorocyclohexane mixture of isomers, is stated by Slade in Chemical Trade Journal and Chemical Engineer for March 16, 1945, at page 279, to be present to the extent of 10l2 per cent in the crude material. Investigators in this country, following the procedure of the Storman application referred to above, have been able to prepare hexachlorocyclohexane compositions containing as high as 16 per cent of 'y by weight in the crude mixture of isomers.

Naturally, since the 'y isomer of hexachlorocyclohexane is the most active principle of any isomeric mixture of hexachlorocyclohexanes, considerable attention has been devoted to the problem of separating the 'y isomer from According to Canadian Chemistry and Process Industries, January 1946, at page 30, Burrage et al. succeeded in isolating a substantially pure 'y-hexachlorocyclohexane isomer and determining that the insecticidal value of hexachlorocyclohexane appeared to be almost entirely due to this form.

The method of Burrage et al., referred to in this publication, is presumably the procedure of British Patent 573,693, granted December 27, 1945, to Burrage and Smart. Up to this time, with the exception of Matthews and van der Linden, the prior art of separating isomeric hexachlorocyclohexanes was substantially as stated in British Patent 573,693:

Hitherto the 'y-isomer has been obtained by continuing the chlorination of the benzene until a slurry of a-benzene hexachloride is formed which is filtered off, and the other isomers present are obtained from the mother liquor by fractional crystallisation. This mother liquor, of course, is saturated with the Ot-lSOIl'lCl, and in addition to the B- and 'y-isomers will contain any small amounts of chlorine-substituted benzene hexachlorides, for example, monochlorobenzene hexachloride, which may be formed. On fractional crystallisation some of the a-isomer crystallises with the 'y-isomer, making the isolation of the latter.

. difficult, and the method troublesome to carry out.

The method of the British patent appears: very similar to that of van der Linden. While producing the y-hexa'chlorocyclohexane isomer in substantially pure form, it is, however, generally unsatisfactory from the point of view of the yield of pure v which can be obtained from a mixture of isomers in a single simple operation. It sometimes requires several repetitions or crystallizations to produce a product of satisfactory 'y content and each single separation usually produces yields of this isomer which are less than desirable for economical operation.

This process, which we have found lacking satisfactory commercial application, is described in the British patent as follows:

We have now found that the 'y-isomer of benzene hexachloride can be recovered from crude benzene hexachloride by extracting it in the cold with a lower aliphatic alcohol such as methanol or ethanol. The a-isomer has but a very small solubility in the lower aliphatic alcohols, as compared with the 'y-isomer, and in addition this solubility appears to be depressed by the presence of the 'y-isomer, so that substantially none of the a-isomer dissolves unless more than enough solvent is used to dissolve all of the Thus a separation of the 'y-isomer from the a can be achieved. The B-isomer or other bodies present in the crude benzene hexachloride Will also be present in the extract containing the 'y-isomer, but on fractional crystallisation of the solution the 'yisomer is deposited first, and can thus be recovered separately. On theother hand, if the presence of these other bodies is not objectionable, the whole of the solids may be recovered from solution together.

Preferably such an amount of the lower aliphatic alcohol is. used to carry outfthe extraction that substantially all of the 'y-isomer is dissolved, and substantially none of the a-isomer. However, if the presence. of small amountsof the ct-isomer is not objectionable rather more of the-alcohol may be used, and a correspondingproportion of the ot-isomer will then be present in the extract.

togetherwith the 'yisomer.

tract without causing the precipitation of substantial" amounts of material other than the 'y-isomer.

1 V The precipitated benzene hexachloride is filtered. oil from the water, dried at 30 C to 35 C. ground, and then treated with between 0.5 and 1.0 times its weight of cold methanol with agitation'for some time, for exam ple 2-4 hours, and the resultant solution filtered off.

In the single example given in the British Patent 573,693, the ratio of methanolto the crude benzene hexachlorideis 0.83 part to 1. In this example, 24 parts of dried benzene hexachloride yielded 0.7 part of the. 'y isomer, and this, calculated on the basis of the to 1-2 percent reported to be present in the crude'by Slade, Chemical Trade Journal and Chemical Engineer, cited above, makes the yield of calculated on the 'y presentv in the crude mixture, between 24 per .cent (if 12' per. cent "y was present) and 29 per cent (if l0per cent was present in the crude). If more than 12 per cent 7 was present in the crude mixture, then the recovery of the. 'y' would appear tobeproportionately less..

Thus, the procedure of the British patent appears to depend upon the following factors:

(1)'Alleged relative insolubility .of the u isomer, in the presence of the 'y isomer.

(2) Alleged possi-bilty of leaving or undi'ssolved while dissolving 'y.

(3) Alleged possibility of precipitating the 'y isomer from asolution: which is substantially free from the a lsomer.

OBJECTS OF THE PRESENT INVENTION It is an obiect of the present invention to provide a process for the isolation of substantially pure 'y-hexachlorocyclohexane from a composition including the v and at least the or isomer of hexachlorocyclohexane. It is. a further obiect of the present" invention to provide such a: process which may be conducted in a simple manner'to give higher yields of substantially pure 'y-hexachlorocyclohexane' than have been previously obtainable by anyknown'process. An additional object of the present invention is to provide such a process"wheresubstantially pure.y-hexachlorocybl'ohexane is crystallizedfrom a solution containing a substantial proportion of the OL-hCXa" chlorocyclohexane isomer. Another objective ofthe invention is the provision of arrows for the isolation ofi substantially pure y-hexachlorocyclohexane from a substantially anhydrous solution which is supersaturated with respect to at least the m-hexachlorocyclohexane isomer. A further obiect of the invention is the provision of sucha process wherein substantially'pure 'y-hexachl'orocyclohexane is. isolated from a methanol solution of the 7; which is supersaturated at least with regard to the on isomer of hexachlorocyclohexane.

THE NEW PROCESS, GENERAL CONSIDERATIONS The teaching of the- British patent is that the. solubility of, the or, isomer of hexachlorocyclohexane. is depressed:

by the presence of the v isomer and that.. if the alcoholic extract does contain some a, in addition tothe 'y, then operation-1s:ln'order-to' obtain a fraction'of high' y con tent, one must repetitively use a limited ratio of solvent to crude hexaclilorocyclohexane calculated to dissolve some 7 present in the crude and avoid extraction of a.

We have now found that, contrary to the teaching of the British. patent, the solubility in methyl alcohol and other solvents of the oc isomer is not depressed by the presence of the 'y isomer. Rather, it is increased. This finding would indicate the impracticality of the British patent procedure and tend to show the undesirability of attempting to use high ratios of solvent to crude hexachlorocyclohexane, since it is not possible to extract from the crudeall of the y isomer therein without dissolving a substantial proportion of the a isomer, amounting to at least one-third of the weight of extracted 'y, particularly in a single extraction.

However, we have also made the critical and unexpectedfinding, again apparently contrary to the British teaching, that, from. an extract of isomer containing a substantial 'proportionof' the or isomer, there can be crystallized afirst fraction of pure "y, This latter findingzis conditioned upon treating the extract of containing the a, as by concentration or chilling,.soasfirst to. form asolution supersaturated withv respect to the a isomer.

The present invention thenincludes: treating asubstantially anhydrous solution of at least the aand 'y-hexachlorocyclohexane isomers to produce a solution, which is supersaturated with respect. to at least the a isomer; and, separating the 7 isomerwhich crystallizes from solution before any substantial amount of a or other hexachlorocyclohexane isomer crystallizes from the solution.

SOLVENTS While the solvent employed in the process as abovedescribed issubstantially anhydrous methanol, it is to be understood that other substantially anhydrous organic.

solvents. may likewise be employed. For example, methyl ethyl ketone, methal, or lower aliphatic monohydric alcohols such as ethanol, n-propanol, or isopropanel may be used. The process, is. equally operative when these other solvents are employed, but methanolisto'be understood as constituting the preferred embodiment'of the invention.

QUANTITY OF EXTRACTING SOLVENT The weight ratio of solvent to crude hexachlorocyclohexane may be aslow as 1.1 to 1.0. The procedure is.

more advantageously conducted when the ratio of solvent to crude hexachlorocyclohexane is at least about 1.1 to 1.0, which is a critical lower limit, since below this ratio the quantity of isomer extracted from the crude falls off sharply, and is most advantageouslyconducted at a ratio of about 1.5-1.6 to 1.0. For example, in the extraction of a crude hexachlorocycl'ohexane mixture containing 13.8 per cent by weight of the isomer,

a ratio of methanol to crude hexachlorocyclohexane of I 017 to 1.0 was found to extract only 4.19 grams of the isomer, while a ratio of 1.1 to 1.0 extracted 6.20 grams of the 'y isomer and a ratio of 1.5 to 1.0 extracted 6.82 grams of the 'y isomer, accompaniedby about 10 per centof the a isomer present in the crude. Since the original crude hexachlorocyclohexaue mixture. contained only-6.9'0 grams of' the 'y isomer, it is apparent that the ratio of"1.5 to 1.0 allowed extraction of 97- per' cent of the 'y isomer, while the other. ratios: of methanol to crude hexachlorocyclohexane allowed correspondingly less efficient extractions. 'It is there preferable to operate the new process with a solvent to crude hexachlorocyclohexane weight'ratio in excess of about 1.1 to 1.0, assuming that the amount of 'y isomer in a crude mixture is betweenv about '10: and- 16 per cent of the totalweight of the hexachlorocyclohexane isomers.

OTHER CONDITIONS FOR EFFECTING SOLUTION Agitation is desirable for obtaining a solution of the 'y and at. least the on isomers of hexachlorocyclohexane. The temperatureat which the solution-is obtained may be varied over a considerable range,.and any convenient or suitable temperature may. beemployedl. Such a. con venient temperature is. at or in the vicinity of room temperature, e. g., 20-3.0. degrees. centigrade. The. solvent employed; for best. results. should. be. substantially. anhydrous. The time required to obtain complete solution will, of course, varysomewhat with thetemperature, vigor of agitation, and quantity of solvent employed, but even at a temperature of about 20 degrees centigrade it is usually unnecessary to agitate the crude hexachlorocyclohexane-and solvent for a period much in excess of about one hour. The solution may be prepared in any suitable apparatus, for example, an iron vessel equipped with an agitator.

At the end of the agitation, or, if agitation is not employed, the period allowed for solution to be obtained, a condition of saturation is established in the extracting solution. This means that the methanol is saturated with respect to the "y and at least the a isomer.

This mixture, consisting of the liquid containing dissolved isomers and the undissolved crude hexachlorocyclohexane, may be filtered at room temperature to remove the undissolved solids. A suitable filter aid may be employed, if desired, butis not always necessary. The filter cake may be washed with an additional amount of solvent to removesubstantially all of the 'y isomer which may be occluded in the filter cake. The wash solution may then be recycled to the extraction step or it may be combined with the filtrate from the previous step, and the combined solution concentrated to supersaturation with regard to the ot isomer in the next step of the process.

The combined filtrates contain substantially all of the on isomer present in the original crude hexachlorocyclohexane mixture, depending, as previously stated, on the ratioof solvent to the crude mixture of isomers, which ratio may be as high as 3.0 to 1.0. This solution is not necessarily saturated with respect to all of the isomers.

CONCENTRATION OF THE SOLUTION The solution may then be heated to its boiling point, or any point in the vicinity thereof which allows the evaporation of solvent therefrom, e. g., about 68 degrees centigrade, when methanol is employed, and about onefifth to two-thirds of the solvent thus evaporated from the solution. This evaporated solvent may be condensed and recovered for reuse in the process. The exact quantity of solvent which is evaporated is immaterial, as it is only necessary to evaporate sufficient of the methanol so that the remaining solution when cooled will be supersaturated at least with respect to the ot-hexachlorocyclohexane isomer. Agitation of the solution during the evaporation step is not essentiahbut is desirable as it increases the rate of evaporation. A convenient apparatus for conducting this evaporation step is an enamellined evaporator provided with an agitator. Vacuum may be used, if desired, to assist the solvent evaporation. The use of vacuum causes the 'y isomer to crystallize in larger crystals, which is sometimes desirable.

CHILLING OF THE SOLUTION COMBINATION RECOVERY A combination concentration and chilling procedure may be followed. Thus, the removal of some solvent may be accomplished by evaporation and a state of supersaturation with respect to the a isomer can then be produced in the solution by cooling to temperatures approximating room temperature.

ILLUSTRATIVE The following examples are given to illustrate the procedure of the present invention, but are in no way to be construed as limiting. The parts are by weight unless otherwise specified.

Example 1 Eighteen hundred parts of crude hexachlorocyclohexane having the following composition:

Isomer: Per cent Alpha 64.8 Beta 9.7

Gamma 13.4

were mixed with.2446 parts of substantially anhydrous methanol in an iron vessel. The mixture was agitated for a period of about one hour at a temperature of 20 degrees Centigrade, and then filtered to remove 1260 parts of undissolved solids, dry weight, consisting almost entirely of the a and B isomers. The filter cake was then washed with 159 parts of methanol to remove 'y-containing mother liquor from the crystals. The original filtrate and methanol wash were charged into an enamellined evaporator provided with an agitator, a condensing system, and heating and/or cooling means, wherein 660 parts of the methanol was evaporated at a temperature of about 68 degrees centigrade and subsequently recovered for further use. The solution remaining in the evaporator was cooled to approximately 20 degrees centigrade with mild swirling agitation over a period of about one hour, at the end of which time the solution was filtered. One hundred forty-nine parts of 96.8 per cent 'yhexachlorocyclohexane were recovered, representing a yield of 63.7 percent of the 234 parts of y-hexachlorocyclohexane in the starting crude mixture.

Example 2 In a manner similar to that given above, 1690 parts of crude hexachlorocyclohexane, having an average hexachlorocyclohexane content of 13.5 per cent, were extracted with 2512 parts of substantially anhydrous methanol. The hexachlorocyclohexane was agitated with methanol for a period of about one hour at 20 degrees Centigrade. At this point, 1183 parts of undissolved solids, dry weight, were removed by filtration, and found to consist of the oz and B isomers. The filter cake was then washed with 159 parts of methanol to remove 7 containing mother liquor. The combined solutions were evaporated at a temperature of about 68 degrees centigrade until 1060 parts of methanol had been evaporated and recovered for further use. The solution was then cooled with gentle swirling agitation over a period of about one hour at approximately 20 degrees centigrade and substantially pure 'y-hexachlorocyclohexane, which precipitated from solution, separated therefrom. In this manner 132 parts v-hexachlorocyclohexane melting at 113 degrees centigrade were-recovered, representing a yield of 57.9 per cent of the 228 pounds of 'y in the starting crude mixture.

Example 3.-Ethanol as solvent.

One kilogram of crude hexachlorocyclohexane containing 13.5 per cent 7 isomer was mixed with one and onehalf kilograms of substantially anhydrous ethanol. (A ratio of 11.1 ethanol to one of 'y isomer in the crude. The mixture was agitated .for one hour at a temperature of 20 degrees centigrade whereupon solubility equilibrium between the ethanol and at least the a and 'y isomer was established. The .mixture was then filtered to remove undissolved solids which Weighed 723 grams when dry, containing 3 per cent 'y or 21.7 grams of y. The filtrate, which weighed 1663 grams, was charged into an evaporator provided for agitation. Here, 767 grams, or about 50 per cent of the ethanol, was evaporated off and recovered for subsequent use. The solution remaining in the evaporator was cooled to about 18 degrees centigrade and maintained at that temperature, with agitation, for a period of one hour, at the end of which time the solution was filtered. There was recovered 61.4 grams of 93.7 per cent 7 isomer. This represents a 42.6 per cent recovery of the 'y isomer present in the crude hexachlorocyclohexane.

Example 4 A filtrate or mother liquor remaining after recovery of 'y isomer, in accordance with our process, but without the wash of the filter cake, was concentrated to a slurry, the solids filtered out, and the solids recycled by mixing with fresh crude hexachlorocyclohexane. After over repetitions of this cycle of steps, the over-all yield of y isomer, of a purity of 93.8 per cent, was found to be about 64.9 per cent.

Example 5 Sixty-six and three-tenths parts of filtrate, made by a methanol extraction of crude hexachlorocyclohexane of Example was repeated, except that the solution was cooled to -l5 degrees centigrade. Two and one-half parts of 98.5 per cent pure 7 isomer was recovered, representing a yield of 42.6 per cent, based on the 7 1n the solution.

Example 7 Example 5 was repeated, except that the solution was cooled to 30 degrees centigrade. Two and six-tenths parts of 96.7 per cent pure 7 isomer, representing a yield of 47 per cent, was recovered.

Example 8 Example 5 was repeated, except that the solution was cooled to 45 degrees centigrade. Three and one-tenth parts of 96.7 per cent pure 1 isomer was recovered, rcpresenting a yield of 53.4 per cent of the 'y isomer 1n the filtrate.

Example 9 Example 5 was repeated, except that the solution was cooled to 62 degrees centigrade. Three and threetenths parts of 97.5 per cent pure 'y isomer was recovered, representing a yield of 59 per cent of the 'y isomer in the solution.

LIMITATIONS Various modifications may be made in the method of the present invention Without departing from the spirit or scope thereof. For example, instead of extracting at room temperature, we may extract at a higher temperature, and if the extraction be carried out under pressure, the temperature may be as high as the boiling point of the solvent.

Similarly, instead of cooling to the very low temperature of the examples, by extracting at a higher temperature, we may bring about supersaturation at room temperatures; also, we do not wish to be limited to cooling to the temperature of Example 9, as the degree of cooling is only limited by the freezing point of the solvent. we contemplate cooling to a temperature of 50 to 100 centigrade degrees below the extracting temperature.

We claim:

'1. The process which includes: mixing one part of crude hexachlorocyclohexane containing at least 10 per cent by weight of the 'y-hexachlorocyclohexane isomer with more than about 1.1 parts of substantially anhydrous methanol until saturation of the methanol occurs with respect to at least the aand 'y-hexachlorocyclohexane isomers; evaporating said saturated solution until at least about 20 per cent of the volume of the solution has been evaporated and the remaining solution is supersaturated with respect to at least the a isomer; and, separating the 'y-hexachlorocyclohexane isomer which crystallizes from In general, 7

solution before any substantial amount of m or other hexachlorocyclohexane isomer crystallizes from solution.

2. The process which includes: extracting with at least 1.1 part by weight of a substantially anhydrous organic solvent a mixture of hexachlorocyclohexane isomers containing a minor proportion of the 'y isomer and a major proportion of other isomers including the u isomer, to produce a solution containing substantially all of the 'y isomer and a substantial proportion of at least the a isomer; separating the undissolved solids; bringing the solution to a state of supersaturation at least with respect to the a isomer; and, recovering the 'y isomer from said supersaturated solution as the first crop of crystals obtained.

3. The process according to claim 2, wherein: the bringing of the filtered extract to a state of supersaturation at least with respect to the a isomer is accomplished by chilling.

4. The process according to claim 2, wherein: the bringing of the filtered extract to a state of supersaturation at least with respect to the a isomer is accomplished by evaporation.

5. The process according to claim 2, wherein: the bringing of the filtered extract to a state of supersaturation at least with respect to the m isomer is accomplished by evaporation and cooling.

6. The process according to claim 2, wherein: the organic solvent is a lower aliphatic monohydric alcohol.

7. The process according to claim 2, wherein: the organic solvent is methyl monohydric alcohol and is employed in a weight ratio in the range between about 1.5 and about 1.6 parts of alcohol per part of crude hexachlorocyclohexane.

8. The process which includes: mixing one part of crude hexachlorocyclohexane containing about 13 percent by weight of the gamma hexachlorocyclohexane isomer with about 1.5 parts of methanol until saturation of the methanol occurs with respect to at least the alpha and gamma hexachlorocyclohexane isomers; separating the undissolved solids; evaporating said saturated solution until about 40 percent of the volume of the solution has been evaporated; cooling the remaining solution until supersaturated with respect to at least the alpha isomer; and, separating the gamma hexachlorocyclohexane isomer which crystallizes from solution before any substantial amount of alpha or other hexachlorocyclohexane isomer crystallizes from solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,438,900 Cooke et a1 Apr. 6, 1948 FOREIGN PATENTS 447,048 Great Britain May 7, 1936 573,693 Great Britain Dec. 3. 1945 OTHER REFERENCES Leeds: "Iour. Am. Chem. Soc., vol. 2, pages 205-7 (1880). 

2. THE PROCESS WHICH INCLUDES: EXTRACTING WITH AT LEAST 1.1 PART BY WEIGHT OF A SUBSTANTIALLY ANHYDROUS ORGANIC SOLVENT A MIXTURE OF HEXACHLOROCYCLOHEXANE ISOMERS CONTAINING A MINOR PROPORTION OF THE Y ISOMER AND A MAJOR PROPORTION OF OTHER ISOMERS INCLUDING THE A ISOMER, TO PRODUCE A SOLUTION CONTAINING SUBSTANTIALLY ALL OF THE Y ISOMER AND A SUBSTANTIAL PROPORTION OF AT LEAST THE A ISOMER; SEPARATING THE UNDISSOLVED SOLIDS; BRINGING THE SOLUTION TO A STATE OF SUPERSATURATION AT LEAST WITH RESPECT TO THE A ISOMER; AND, RECOVERING THE Y ISOMER FROM SAID SUPERSATURATED SOLUTION AS THE FIRST CROP OF CRYSTALS OBTAINED. 